Correct option is D
A key field (primary key or candidate key) uniquely identifies each record in a table.
Its defining property is uniqueness—no two rows may share the same key value.
This guarantees unambiguous retrieval, updates, and deletion of specific records.
Keys may be numeric or text; data type is not the defining feature.
Duplicate or nullable values violate primary key rules.
Therefore, the essential characteristic is that the key must be unique.
This guarantees unambiguous retrieval, updates, and deletion of specific records.
Keys may be numeric or text; data type is not the defining feature.
Duplicate or nullable values violate primary key rules.
Therefore, the essential characteristic is that the key must be unique.
Important Key Points
- Primary Key: Unique and non-null; one per table.
- Candidate Keys: All fields (or combinations) that could serve as a primary key; each must be unique.
- Alternate Key: Candidate key not chosen as the primary key.
- Surrogate vs Natural: Keys can be artificial (e.g., auto-increment ID) or real-world (e.g., ISBN).
- Indexing: Keys are typically indexed to speed lookups and enforce uniqueness.
- Referential Integrity: Foreign keys reference primary/unique keys in related tables.
Knowledge Booster
- Why not (a) text? Keys can be any data type (int, UUID, text); type doesn’t define a key.
- Why not (b) duplicate? Duplicates are disallowed; duplicates break identity of rows.
- Why not (c) a name? Names are often not unique; thus poor primary keys unless constrained.
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